Musnad Abu Ya'la al-Mawsili 3:198

Abu Khaythama narrated to us, Abu Mu'awiyah narrated to us, al-A'mash narrated to us, from Mundhir, from Muhammad ibn al-Hanafiyyah, from Ali, who said: I was a man who produced much madhy (pre-seminal fluid), and I was too shy to ask the Messenger of Allah, peace and blessings of Allah be upon him, about the madhy. He said: So I told al-Miqdad to ask the Messenger of Allah, peace and blessings of Allah be upon him, about the madhy. He said: So he asked him, and the Messenger of Allah, peace and blessings of Allah be upon him, said: "Regarding it, there is wudu (ablution)."